1. Field of the Invention
The present invention relates generally to fuel cells which convert the latent chemical energy of a fuel into electricity directly through electrochemical reactions and, more particularly, is concerned with fuel cell plates employing skewed process channels for providing a more uniform distribution of the compression load imposed on the fuel cells in a stack thereof.
2. Description of the Prior Art
One common fuel cell system includes a plurality of subassemblies which, except for the top and bottom subassemblies, each include two bipolar plates between which is supported two gas electrodes, one an anode and the other a cathode, and a matrix with an ion-conductive electrolyte, such as phosphoric acid, between the anode and cathode electrodes. The subassemblies, herein referred to as fuel cells, are oriented one atop another and electrically connected in series (alternate electron and ion paths) to form a fuel cell stack. The top end plate of the top subassembly and the bottom end plate of the bottom subassembly are each half-bipolar plates. Representative examples of such fuel cell system are disclosed in U.S. Pat. Nos. to Kothmann et al (4,276,355; 4,342,816), Kothmann (4,292,379; 4,324,844; 4,383,009) and Pollack (4,366,211) which, with the exception of U.S. Pat. Nos. 4,342,816 and 4,383,009, are assigned to the assignee of the present invention.
Process gases, such as a fuel and an oxidant are supplied respectively to the anode and cathode electrodes via manifolds attached to the stack and channels defined in the bipolar plates. The fuel in the form of hydrogen atoms when supplied to the anode dissociates into hydrogen ions and electrons. The electrons are transmitted from the anode electrode across the bipolar plate to the next cell's cathode electrode, while the hydrogen ions migrate directly through the acidic electrolyte to its cell's cathode, where they react with electrons from another anode and oxygen to form water. This is repeated through the stack out through the ends where the electron transfers from the last anode to the last cathode as the other end of the stack is through the external circuit where useful work is produced.
The bipolar plates of the fuel cells basically function to structurally support and physically separate the individual fuel cells and provide cavities for the process gases to access the anode and cathode electrodes where the electrochemical reaction occurs. The plates are normally rectangular or square in shape with a series of generally parallel channels formed in both top and bottom surfaces thereof. Recently, several problem areas have been recognized with respect to the process gas flow channel configurations found in conventional fuel cell bipolar plates.
First, a problem exists with respect to inadequate process gas flow distribution to the fuel cells in the lower portion of the fuel cell stack as compared to in the upper portion. Conventional process channels in bipolar plates tend to be relatively short and have few directional changes. As a consequence, process gas pressure drop in the horizontal direction in traversing the plate channels within a given fuel cell is small in comparison with the pressure drop in the stack distribution manifold and piping system in the vertical direction. This results in an inherently poor flow distribution of the process gases to the fuel cells, more flow to the cells in the upper portion than in the lower portion of the fuel cell stack. One approach to reduce this problem involves restricting the flow of process gases between the supply piping and the manifold cavities adjacent to the stacks to artificially increase the pressure drop in the parallel fuel cell circuits between the supply and exit piping and thereby improve flow distribution. However, this approach complicates the stack design, increasing the number of parts and, thus, cost.
Second, a problem exists with respect to non-uniform distribution of a compressive load carried across the fuel cells which maintains the fuel cells operatively assembled together in the stack. Depending upon the particular desired configuration for process gas flow through the fuel cell (i.e., crossflow, countercurrent, concurrent, or some combination thereof), the flow channels in one surface of a bipolar plate may run either perpendicular or parallel to the flow channels in the other surface of the plate. Thus, with respect to any given fuel cell, the flow channels in the lower surface of the top plate will run either perpendicular or parallel to the flow channels in the upper surface of the bottom plate. Also, a typical fuel cell includes a sealing component or gasket at the outer edges of the cell that fits between the plates and forms a boundary to separate the process gases from each other and from the external environment.
The areas where the lands or ribs defining the channels in the upper and lower plate surfaces overlap represent the areas of contact (through the electrodes and electrolyte matrix) between the cell plates through which the compressive load is transferred. In regions where the ribs cross perpendicular to each other, the contact area between the opposing plate surfaces remains nearly constant, varying only to the extent that manufacturing tolerances affect the width of the ribs. In regions where the ribs are parallel to each other, the contact area is also a function of manufacturing tolerances affecting the width of the ribs; but in addition, variations in channel to channel placement, molding shrinkage, edge machining and assembly alignments (plate to plate) also affect contact area. The net result is that parallel rib contact area varies much more than perpendicular rib contact area.
The cell components are quite susceptible to damage and premature failure if the contact areas are not controlled within certain limits, to limit contact stress and related cell strain. Any significant thickness variations associated with the plates compound the problem by providing local areas of higher or lower stress. A low stress limit must be maintained to insure minimum electrical contact resistance for uniform cell performance. To optimize uniform cell performance it is desirable to have both parallel and perpendicular contact areas nearly the same so the compression load is carried uniformly across the cell.
Consequently, a need still exists for improvements in the configuration of the fuel cell process gas flow channels in order to alleviate the problems of inadequate flow distribution across the fuel cells in a stack thereof and non-uniform compression loading of the fuel cells.